Point defects, ferromagnetism and transport in calcium hexaboride

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Abstract
The formation energy and local magnetic moment of a series of point defects in CaB6 are computed using a supercell approach within the local density approximation to density functional theory. Based on these results, speculations are made as to the influence of these defects on electrical transport. A tentative explanation is given for the strong dependence of the latter on the growth procedure. It is found that the substitution of Ca by La does not lead to the formation of a local moment, while a neutral B6 vacancy carries a moment of 2.4 Bohr magnetons, mostly distributed over the six nearest-neighbour B atoms. A plausible mechanism for the ferromagnetic ordering of these moments is suggested. Since the same broken B-B bonds appear on the preferred (100) cleavage planes of the CaB6 structure, it is argued that internal surfaces in polycrystals as well as external surfaces in general will make a large contribution to the observed magnetization.

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